They do work, and here’s why: http://www.highpath.net/highpath/cycles/ovals/01.html#how
However, in my experience of using them on a singlespeed, they are not great for this application: a smaller round ring would be better. For multiple rear cogs, they will get you through the dead spots better, and give you an edge when technical climbing. They feel weird for all of about five minutes, after which round rings feel odd and ‘dead’ like Biopace.
Unfortunately, Chris Bell is not making them any more, so I’ve been looking for another source.
@tazzymtb: Are Goldtec still making them, and would they be willing to make custom ones do you know?